A switched-mode power converter (also referred to as a “power converter” or “regulator”) is a power supply or power processing circuit that converts an input voltage waveform into a specified output voltage waveform. DC-DC power converters convert a direct current (“dc”) input voltage that may be derived from an alternating current (“ac”) source by rectification into a dc output voltage. Controllers associated with the power converters manage an operation thereof by controlling conduction periods of power switches employed therein. Some power converters include a controller coupled between an input and output of the power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”) to regulate an output characteristic of the power converter.
Typically, the controller measures the output characteristic (e.g., an output voltage, an output current, or a combination of an output voltage and an output current) of the power converter, and based thereon modifies a duty cycle or an on time (or conduction period) of a power switch of the power converter to regulate the output characteristic. To increase an efficiency of a flyback power converter, a capacitor is coupled across a power switch to limit a voltage of the power switch while a transformer of the power converter is reset when the power switch is turned off. A flyback power train topology may be configured as a quasi-resonant flyback power converter.
In a common application of a flyback power converter, an output current of the power converter is regulated. With conventional design approaches, however, it is difficult to achieve quasi-resonant power converter operation and, at the same time, regulate an output current of the power converter. In one conventional approach, an on time of a diode on a secondary side of the power converter is sensed and a peak value of primary current is held constant, the output current is kept constant by controlling an off time of a power switch on a primary side of the power converter. This process may defeat quasi-resonant switching operation of the power converter.
In another approach, an output current is sensed and a power switch on a primary side of the power converter is controlled employing an optocoupler to transmit a signal of the secondary side of the power converter to a controller referenced to the primary side of the power converter. This approach increases power converter cost due to the presence of the optocoupler. In yet another approach, a regulation of an output current is implemented through the controller by calculating an output current employing an average of input current and a duty cycle of a power switch on a primary side of the power converter. This approach preserves quasi-resonant switching without the need for an optocoupler, but requires a complex calculation in the controller.
In a switched-mode power converter, it is generally beneficial to limit a peak current in a primary winding of a magnetic circuit element (or device) such as a power transformer or an inductor. This prevents magnetic saturation in the magnetic circuit element, to protect a power switch employed therein, or to limit a maximum level of output power from the power converter. If a peak current in a winding of the magnetic circuit element is not limited or otherwise controlled to a constant level, this can have an unwanted effect on output of the power converter (e.g., an output ripple can increase), and a primary-controlled output current limit can exhibit an unwanted level of variation.
It is common practice in the design of a switched-mode power converter to use a comparator to limit a peak current to a desired current limit in a primary winding of the magnetic circuit element. The comparator sends a signal to control logic when a voltage at a shunt resistor or other current-sensing circuit element becomes higher than a reference voltage. Then the control logic switches off a power switch. The comparator, the control logic and the power switch, however, operate with inherent delays that can be variable as a function of the operating environment and sensed voltages. Due to these inherent and variable delays, current in the power switch and the magnetic circuit element continues to rise with the result that the peak of the current becomes higher than the desired current limit by a variable amount that is generally dependent on an input voltage to the power converter. A higher input voltage generally produces a higher current difference between the peak of the current and the desired current limit.
Thus, a peak current limiter that limits a peak current of a power converter that produces a constant level thereof still presents unresolved design challenges. Accordingly, what is needed in the art is a design approach and related method to implement a controller that determines and limits a peak current for a power converter without compromising end-product performance and that can be advantageously adapted to high-volume manufacturing techniques.